The deputy secretary-general of the United Nations was last night accused of making "a very, very grave mistake" after calling the Bush administration hypocrites who were feeding a right-wing anti-UN frenzy in middle America.

Washington's ambassador to the UN responded with undisguised fury to a speech by Mark Malloch Brown, the deputy secretary-general, in which he accused Washington of using the international body "almost by stealth as a diplomatic tool" while failing to defend it at home.

"Much of the public discourse that reaches the US heartland has been largely abandoned to its loudest detractors, such as Rush Limbaugh and Fox News," Mr Malloch Brown said in a speech in New York on Tuesday. Depending on the UN while tolerating "too much unchecked UN-bashing and stereotyping" was "simply not sustainable", he said. "You will lose the UN one way or another."

John Bolton, the US envoy and an outspoken critic of the UN, called the comments "a very, very grave mistake". He said he told the secretary-general, Kofi Annan, yesterday morning: "I've known you since 1989, and I'm telling you, this is the worst mistake by a senior UN official that I have seen in that entire time." He called on the secretary-general to repudiate the speech.

Tensions between the UN and George Bush's White House have been simmering since the war in Iraq, but they also encompass deep splits over the international criminal court and the new human rights council, whose formation the US was one of only four states to oppose. But the diplomatic tradition according to which UN officials do not publicly attack specific member states has a longer history still.

Washington was angered by Mr Malloch Brown's references to middle America, and the influence upon it of conservative commentators such as Mr Limbaugh. Mr Bolton said the speech demonstrated a "condescending, patronising tone about the American people. Fundamentally and very sadly, this was a criticism of the American people, not the American government, by an international civil servant. It's just illegitimate."

The deputy secretary-general, a Briton who was previously head of the UN development programme, called his speech "a sincere and constructive critique of US policy toward the UN by a friend and admirer". But he may have felt liberated to speak his mind: his term of office ends in December, at the same time as Mr Annan's, and he is understood not to be planning to stay at the UN.

As speculation over Mr Annan's successor mounts, some reports have suggested Tony Blair is angling for the job - and that a speech he gave at Georgetown University two weeks ago, in which he outlined his ideas on UN reform, constituted a job application of sorts.

Mr Malloch Brown also used his speech to defend 18 peacekeeping missions around the world and to criticise continuing efforts by the US to use its leverage - as by far the UN's largest funding source - to press for more rapid reforms.

The UN needed to be overhauled, he acknowledged, but "in recent years the enormously divisive issue of Iraq and the big stick of financial withholding have come to define an unhappy marriage", he said. Other nations had the perception that the US adopted "maximalist positions" in diplomatic negotiations, he added, rather than seeking compromise.

A UN official, speaking on condition of anonymity, told the Guardian the speech was intended to be "a warning call" about a broader crisis. On many issues, including UN reform, "what the US is doing is absolutely right from our point of view", the official said. "It's just that almost nobody else, in the current environment, believes them." The state of public discourse about the UN in America meant that "the rest of the world thinks that the US has a hidden agenda, or is trying to use the process to manipulate the UN".

A senior UN official, Shashi Tharoor, said on Tuesday that the UN risked having to shut down if the US and Japan carried out a threat to withhold funding because management reforms had not been sufficiently implemented.